1. Field of Invention
The present invention relates to a dual mode radar system with an active aperture phased array antenna; and more particularly, to such a dual mode system utilizing a solid state active aperture phased array antenna system and related method. Although suitable for the transmission of radiant energy having different average and peak power, the antenna system of the present invention is particularly advantageous for dual mode operation where radar pulses and a continuous wave (CW) are transmitted alternately by the same antenna, and will be described in connection therewith.
2. Discussion of Related Art
A phased array antenna is made up of a multitude of individual radiating elements which are excited from a common signal source to maintain phase coherence. A beam is formed from the superposition of the radiation by each of the individual elements. As is well known, by changing the phase of the signal applied to each element the direction of the beam is varied.
Phased array antennas may be either of the passive aperture type or the active aperture type. In passive aperture arrays, each radiating element merely includes phase shifters for changing passively the microwave energy. In active aperture arrays, each radiating element includes an associated solid state module having a power amplifier stage for amplifying the microwave energy, preferably subsequent to phase shifting. Active aperture solid state antennas are advantageous in that they permit the forming of a high power beam by power combining in free space a large number of low power beams; and are also advantageous for multi-mode operation; that is, where the same radiation elements of the antenna array are used for different types or modes of transmission and reception. Multi-mode operation, of course, minimizes the number of microwave apertures and radiating elements required, by permitting the use of the same aperture for more than one function.
In multi-mode operation, it is often desirable to operate in a manner that includes both pulsed radar and continuous wave (CW) electronic countermeasure modes, for example. However, as far as is known, prior to the present invention, it was not possible to optimize the active apertures for both the pulsed radar and continuous wave modes because of the widely different duty cycles of their waveform. The active microwave elements or amplifiers tend to be peak power limited, while the power sources tend to be average power limited.
This situation required that one of two compromises be observed when both operating modes must be served. One compromise involved providing microwave modules and a power supply having the capacity to operate all of the antenna elements at the required power in the CW mode, thus providing maximum Average Effective Radiated Power (AERP) for the CW electronic countermeasure operation. For the pulsed mode, the AERP was then necessarily reduced to a fraction of the AERP in the CW mode, corresponding to the duty cycle of the pulsed mode. Thus, for a pulsed mode having a typical duty cycle of twenty-five percent, the AERP is reduced to twenty-five percent of the AERP in the CW mode. For the other compromise, microwave modules and a power supply were utilized that had the capacity to operate at maximum AERP in the pulsed radar mode; and then for (CW) operation, because of the average power limitation of the power source, only a fraction of the total available individual elements were energized. Although this compromise provided the same average radiated power for both modes, the usable aperture of the antenna was reduced by a percentage corresponding to the number of deenergized elements. This, of course, reduced the Actual Average Effective Radiated Power (AERP)for the CW mode below that of the pulsed radar mode. Therefore, depending upon which compromise situation was used, a significantly less AERP was available for either the pulsed radar or CW mode.